Method of casting parts having dimensionally critical apertures

ABSTRACT

A process for making a product having a dimensionally critical aperture extending therethrough comprises placing a sized elongated pin in a predetermined position through a mold cavity. A body is cast in the mold cavity for use in a lost-wax process. The body is then reproduced in metal by the lost-wax process. The pin is carried along with the cast body so that the body constructed in metal bears the pin at the location of the intended aperture. The pin is thereafter dissolved from the metal body.

United States Patent Arnold et al. 51 Dec. 26, 1972 54] METHOD OF CASTING PARTS 3,368,244 2/1968 Mueller ..l64/45 HAVING DIMENSIONALLY CRITICAL 1,544,929 7 1925 Pack ..l64/l32 x 2,687,278 8/1954 Smith et al ..l64/l32 X [72] Inventors: George R. Arnold; Raymond S.

Rosiello,-both of Glendora, Calif.

[73] Assignee: Ormco Corporation, Glendora,

Calif.

[22] Filed: Jan. 18, 1971 [21] Appl.No.: 107,032 1 s21 U.S. c1 ..l64/45, 164/132 [51] Int. Cl ..B22c 7/02 7 [58] Field of Search ..l64/45, 132

[56] References Cited UNITED STATES PATENTS 7 3,596,703 8/1971 Bishop et a1 ..l64/l32 Primary Examiner-J. Spencer Overholser Assistant Examiner-John E. Roethel Attorney-Angus 81. Mon

[5 7] ABSTRACT A process for making a product having a dimensionally critical aperture extending therethrough comprises placing a sized elongated pin in a predetermined position through a mold cavity. A body is cast in the mold cavity for use in a lost-wax process. The body is then reproduced in metal by the lost-wax process. The pinis carried along with the cast body so that the body constructed in metal bears the pin at the location of the intended aperture. The pin is thereafter dissolved from the metal body.

5 Claims, 9 Drawing Figures METHOD OF CASTING PARTS HAVING DIMENSIONALLY CRITICAL APERTURES This invention relates to a process for making orthodonic brackets of the class having precision apertures extending through the bracket through which a ligature wire may be laced.

Edgewise orthodonic brackets are widely used in corrective dentistry. Generally, they are spot welded to a metal band which embraces the tooth and a rectangular ligature wire is laced through an aperture in the member. The wire extends between two such members so as to control and brace the positioning of the teeth by being ligatured thereto, and drawn the length. The wire itself is rectangular in cross-section and has very critical dimensions which must. be critically matched by the aperture through the bracket. Even small tolerances result in unacceptable variances and can result in uncontrolled tooth displacement, so a nearly zero tolerance is necessary. Hence, the aperture in the bracket is ordinarily held to tolerances of plus 0.0002, minus 0.0000 inches. Sometimes a second ligature wire is used so that the bracket must include a second precisely sized aperture for the second wire.

The orthodonic bracket of this class is an extremely difficult product to construct especially in view of the closely dimensioned apertures which are used to retain the ligature wires. Prior to this invention, brackets had been constructed by using expensive and difficult milling, drilling and undercutting processes, resulting in high scrap rates, expensive tooling, and numerous inspections of interim configurations.

The present invention is concerned with a casting technique for casting precision apertures. By using the process according to this invention, the cost of manufacturing the article is substantially reduced. The

process requires only a few simple, easily performed steps, and the process offers the use of a wide variety of materials of construction which have not heretofore been useful for making this type of product. By way of example, the process of this invention permits the construction of a bracket with 300 Series stainless steel, such as 304 stainless steel, which can be cast but not readily machined. The functional advantage of the 300 Series stainless steel is that it is more ductile than the rod or bar type stainless steel which had been previously used because of their machinability but which had not been significantly ductile. Ductility is a desirable property in orthodonic brackets of the type involved herein because ductility reduces the probability of breakage from shock or other forces. The ductility of castable alloys that can be used in the process in accordance with the present invention is significantly greater than that of alloys which had heretofore been used.

Other resulting advantages from the materials which can be used with this process include an improved weldability of the bracket to the band encasing the tooth. It has been determined that less pressure and heat are required to produce a stronger weld between the band and the bracket than are needed to produce a less reliable weld with materials which had heretofore been used. In this connection, persons skilled in the art will understand that alloying elements such as sulphur and selenium which have been provided to improve machinability, interfere with the function of weldability. Thus, brackets manufactured in accordance with the present invention are more readily welded because they do not include alloys in the material which are deleterious to the weld and to welding generally.

The process of this invention includes casting a body in a mold having a precisely sized pin positioned in the mold across the mold cavity in a position matching that of the intended aperture. The size of the pin is matched to the size of the intended aperture. The body, ordinarily constructed of suitable plastic or wax to be lost in a lost-wax process, is removed from the mold, carrying the pin with it. The body and pin are encased in a frangible casting jacket, and the body is then' lost in a lost-wax process leaving a cavity matching the size and shape of the intended product and having the pin extending across the cavity and held bythe walls of the casting jacket in the location of the intended aperture. This jacket is then filled with a castable alloy to produce a duplicate part containing the pins in the position of the intended precision aperture. Thereafter, the pins are dissolved out leaving the aperture, and the part is metal-finished. 7

According to an optional but preferred feature of the invention, groups of parts are cast together so that the part may be mass produced.

In accordance with another preferred feature of the invention, the mold includes at least one parting line in the plane of the pin, so that the body to be lost in the lost-wax process may be removed from the mold without damaging the mold and the mold may be reused.

The above and other features of this invention will be more fully understood from the following detailed description and the accompanying drawings, in which:

FIG. 1 is an end view elevation of a bracket produced by the process according to the present invention;

FIG. 2 is a top view elevation, partly in cut-away cross-section, of the bracket illustrated in FIG. 1, FIG. 1 being taken at line 11 in FIG. 2;

FIG. 3 is a side view showing a tooth held in a band incorporating an orthodonic bracket illustrated in FIGS. 1 and 2;

FIG. 4 is a perspective assembly view of a portion of a mold used in the process for constructing the orthodonic bracket illustrated in FIGS. 1 and 2;

FIG. 5 is a plan view of one portion of the mold illustrated in FIG. 4 taken at line 5-5 in FIG. 4;

FIG. 6 is a section view of the mold illustrated in FIG. 4 showing the process of constructing a body for use in the lost-wax process in accordance with the present invention;

FIG. 7 is a section view showing the body used in the lost-wax process encased by a jacket in accordance with the process according to the present invention;

FIG. 8 is a section view of the jacket and body shown in FIG. 7 taken at line 8--8 in FIG. 7; and

FIG. 9 is a perspective view of the device constructed by means of the lost-wax process.

FIGS. 1 and 2 illustrate an orthodonic bracket 10 constructed by the process in accordance with the presently preferred embodiment of the present invention. Bracket 10 includes a body 11 having a somewhat elliptical cross-section and an integral base pad 12. Ordinarily, base pad 12 includes wings 13, l4, l5 and 16 which are bent slightly down and away from the body 11 to assure conforming of the bracket to the curvature of tooth and to assure proper contact of the wings with a band. about a tooth for welding the bracket to the band. Apertures 17 and 18 are disposed longitudinally through body 11 and are precisely sized as to receive and hold ligature wire therein. Preferably, and as illustrated in FIG. 1, the widest point of the cross-section of body 11 is in the same plane as the diameter of aperture 18 and the upper surface of aperture 17.

As illustrated particularly in FIG. 3, bracket is welded to band 19 which encases tooth 20; preferably the welds being accomplished where wings 13, 14, 1S, and 16 contact the band. Ligature wire 21 is laced through apertures 17 and/or 18 between several brackets attached to several bands encasing several adjacent teeth to align the teeth and hold them in position. Hence, with the bracket attachedtothe bands and laced together with ligature wire, the assembly forms an orthodonic brace. 7

It is important to orthodonic medicine that the ligature wire 21 be sized relative to aperture 17 or 18 as to make a snug fit therewith, so that the teeth are not permitted to move appreciably after being fixed in position by the orthodonic brace. Thus, it is generally considered that ordinary tolerances for apertures 17 and 18 must be within plus 0.0002 inches and minus 0.0000 inches. The present invention is concerned with a method of constructing the bracket, particularly by casting techniques, so that the brackets may be mass produced economically and will have properties more desirable than bracket heretofore produced by conventional techniques. g

In accordance with the present invention, and as illustrated in FIG. 4, a mold is provided having mold sections 22, 23, and 24. Mold section 22 includes cavity conforming to the uppermost portion of the body 11 of bracket 10 illustrated in FIGS. 1 and 2. Also, mold sec- If desired, the mold may include a plurality of cavities along the length of the mold for manufacturing several parts during a single casting operation. In this regard, channel 37 extends longitudinally along the length of the mold in mold section 24 and is connected to each mold cavity by means of aperture channel 38.

In the use of the mold illustrated in FIGS. 4 and 5,'a suitable material to be lost in a lost-wax process, such as polystyrene plastic, is cast into the mold cavities to fill the mold cavities. Polystyrene isquite suitable for this purpose because it is dimensionally stable when cured, and will decompose to gas when strongly heated. FIG. 6 is a crossfsection of the mold having the polystyrene plastic-disposed in the mold cavity. As illustrated in FIG. 6, the polystyrene plastic forms a body 39 in the mold sections. Pins 27 and 31 extend through body 39 so that upon curing of the plastic forming body 39, the pins will be embraced within the body. To affectuate removal of body 39 from'the mold, the mold sec- I tions are disassembled and the body is removed.

tion 22 includes an elongated cavity 26 adapted to receive a half-cylindrical portion of pin 27. Pin 27 is accurately sized to within close tolerances relative to intended aperture 18 in body 11. Mold section 23 in cludes an internal cavity 28 adapted to form the remainder of body 11 of bracket 10 and includes elongated cavities 29 and 30. Cavity 29 is adapted to receive a half-cylindrical portion'of pin 27 and cavity 30 is adapted to receive rectangular pin 31. Pin 31, like pin 27, is accurately sized to the dimension of intended aperture 17 in body 11. Mold section 24 includes cavity 32 adapted to form the base pad 12 of bracket 10. The parting line between mold sections 22 and 23 is the plane containing the widest portion of body 11 and the diameter of pin 27 and one surface of pin 31. The parting line between mold sections 22 and 23 is on the plane containing the junction of body 11 and base pad 12.

When planar surfaces 33 and 34 of mold sections 22 and 23 are in contact with each other, and when planar surfaces 35 and 36 of mold sections 23 and 24 are in contact with each other, the cavity formed by the cavities in the respective mold sections match to the size and shape of the part to be manufactured. Furthermore, pins 27 and 31 extending through the mold cavity match to the size, shape and location of the intended apertures 17 and 18. Preferably, the positions of the mold sections are aligned by means of pins and pinapertures (not shown). Clamps (not shown) may be utilized to hold the mold sections together.

Upon removal of body 39 from the mold, the part to be lost in the lost-wax process appears as a plurality of bodies 39- each bearing pins 31 and 27 and connected by means of polystyrene rod 41 to base 40. Hence, the completed body for use in the lost-wax process is similar in appearance to the part shown in FIG. 9.

After the polystyrene body within the mold formed by mold sections 22, 23 and 24 has completely cured, and the part to be lost in the lost-wax process has been removed from the mold, the part bearing body 39 is encased in a pliable ceramic jacket 42 which in turn is baked to form a rigid mold. When subjected to the intense heat of the baking process, and after the jacket has baked to a rigid mold, body 39 decomposes and gasifies and is expelled through the aperture left by channel 40 of the body thereby leaving an internal wall within jacket 42 which exactly duplicates the shape of the body. Pins27 and 31 are held within the walls of jacket 42 and extend across the cavity left by the gasified body. The polystyrene plastic is burned out of the ceramic jacket at a controlled temperature of about 1200 F. At this temperature, the polystyrene plastic completely gasifies and is removed from the cavity within the jacket. It is important to the present invention that the application of heat be carefully controlled and that the temperature does not significantly fluctuate so that deflection of pins 27 and 31, due to thermal expansion is minimized and the environment is controlled so that the surface of the pins will not oxidize. It is preferable that the heating process be carried out in an inert atmosphere to prevent oxidation of the pins.

Next, the cavity in the jacket is filled with molten metal such as 304 stainless steel which completely fills the cavity and embraces and surrounds pins 27 and 31. By way of example, the molten stainless steel may be cast in a spin mold under centrifugal force at a temperature of about 1400 F to assure complete filling of the mold cavity with stainless steel. The jacket and the product are thereafter cooled, the jacket is cracked open and broken off thereby leaving the product 10 connected by means of posts 43 to base 44. (See FIG. 9). Product 10 is an exact duplicate of the internal cavity formed from body 39 and is identical to the completed orthodonic bracket, except for final milling and removal of pins 27 and 31.

Pins 27 and 31 are preferably constructed of a low carbon steel, preferably containing about 0.02 percent carbon. The low carbon steel is chosen due to its ability to withstand temperatures of 1400 F and more and because it will dissolve in nitric acid which does not affect stainless steel. The product illustrated in FIG. 9 together with connecting cylinders 43 and base 44 are washed for about an hour in a 20 percent solution of nitric acid, heated to approximately 200 F, to dissolve away the carbon steel. The 20 percent solution of nitric acid will not attack or affect 300 Series stainless steel but will dissolve the low carbon steel pins. After about of jacket material and nitric acid and may be milled to accurately size the outer portions of the bracket.

Upon completion of the part and removal of the pins, the part may be completed by cutting the post 43 free from base pad 16 and thereafter milling and grinding the part to its final size. If desired, wings 13, 14, and 16 of base pad 12 may thereafter be bent at a slight angle to achieve the final shape illustrated in FIGS. 1 and 2.

The present invention thus provides a method of casting parts having precisely dimensioned apertures. The process according to this invention offers the advantage of permitting mass production of accurately sized parts and the production of parts using materials not heretofore used for such parts. Thus, orthodonic brackets constructed by the process according to the present invention may be fabricated from stainless steel not heretofore used for such brackets offering more desirable characteristics than those not heretofore present in prior brackets. The process according to the present invention is effective and may be accomplished without the inspection of interim operations and parts. The process permits the duplication of additional brackets with a high degree of reliability.

This invention is not to be limited by the embodiment shown in the drawings and described in the description, which is given by way of example and not of limitation, but only in accordance with the scope of the appended claims.

What is claimed is:

1. A process of making a product having a dimensionally critical aperture extending therethrough comprising:

placing an elongated pin in a predetermined position through a cavity of a mold, the mold cavity being so sized and shaped to closely match the size and shape of the product to be made, the pin being so sized and shaped to closely match the size and shape of the intended aperture, the pin being positioned in the mold cavity in the location closely matching that of the intended aperture;

supplying a liquid substance into the cavity completely filling the cavity;

permitting the liquid substance to become solid h2i&ll ?i1%vin the solidified substance and the pin from the mold cavity;

encasing in a snug fitting manner the solidified substance with a thin layer of sheet material;

removing the solidified substance from within the layer of sheet material;

supplying a liquid metal within the layer of sheet material;

maintaining the temperature of the liquid metal below the temperature at which would result in melting of the elongated pin;

causing the liquid metal to become solid;

physically removing the layer of solid material from around the solidified 'metal; and

removing the elongated pin thereby resulting in the forming of the dimensionally critical aperture within the molded product.

2. The method as defined in claim 1 wherein the pin is constructed of a material to withstand high temperatures while yet being readily susceptible to being dissolved chemically, the step of removing the elongated pin is accomplished by dissolving the pin chemically.

3. The method as defined in claim 1 wherein the liquid substance supplied within the mold cavity comprises heated wax, after the wax had been solidified the removing of the solidified wax from within the layer 'of solid material is accomplished by reheating the wax causing such to become liquid and pour from within the layer of solid material.

4. The method as defined in claim 3 wherein the melting of the wax to permit such to be removed from 

1. A process of making a product having a dimensionally critical aperture extending therethrough comprising: placing an elongated pin in a predetermined position through a cavity of a mold, the mold cavity being so sized and shaped to closely match the size and shape of the product to be made, the pin being so sized and shaped to closely match the size and shape of the intended aperture, the pin being positioned in the mold cavity in the location closely matching that of the intended aperture; supplying a liquid substance into the cavity completely filling the cavity; permitting the liquid substance to become solid about the pin; physically removing the solidified substance and the pin from the mold cavity; encasing in a snug fitting manner the solidified substance with a thin layer of sheet material; removing the solidified substance from within the layer of sheet material; supplying a liquid metal within the layer of sheet material; maintaining the temperature of the liquid metal below the temperature at which would result in melting of the elongated pin; causing the liquid metal to become solid; physically removing the layer of solid material from around the solidified metal; and removing the elongated pin thereby resulting in the forming of the dimensionally critical aperture within the molded product.
 2. The method as defined in claim 1 wherein the pin is constructed of a material to withstand high temperatures while yet being readily susceptible to being dissolved chemically, the step of removing the elongated pin is accomplished by dissolving the pin chemically.
 3. The method as defined in claim 1 wherein the liquid substance supplied within the mold cavity comprises heated wax, after the wax had been solidified the removing of the solidified wax from within the layer of solid material is accomplished by reheating the wax causing such to become liquid and pour from within the layer of solid material.
 4. The method as defined in claim 3 wherein the melting of the wax to permit such to be removed from within the layer of solid material is accomplished in an inert atmosphere.
 5. The method as defined in claim 1 wherein the step of encasing is accomplished by applying a pliable ceramic about the solidified substance, baking the pliable ceramic to cause such to assume the layer of sheet material. 